This application claims the priority benefit of Japanese application serial no. 2001-235852, filed on Aug. 3, 2001.
1. Field of the Invention
This invention relates in general to a fixing device and an image forming device, which can be used in a facsimile device, a printer, a copier, or a multi-function machine with the above functions.
2. Description of Related Art
FIG. 7 is a side view showing an internal structure of a conventional fixing device. The item 100 is a fixing roller. A pressing roller 101 is pressed to be in contact with the fixing roller 100. A fixing heater 102 is arranged inside the fixing roller 100 for heating the fixing roller 100. A temperature detecting device 103 is used for detecting the temperature of the fixing roller 100. A separation claw 104 is used for separating the recording paper wound on the fixing roller 100 from the fixing roller 100. A cleaning pad 105 is in contact with the fixing roller 100 for cleaning the fixing roller 100. A cleaning pad holder 106 is used for supporting the cleaning pad 105. A cleaning pad invasion-regulating unit 107 is installed at a downstream side of the rotational direction of the fixing roller 100 with respect to the cleaning pad 105. An entrance guiding plate 108 is installed at a recording paper entrance of the fixing device 100. A paper-ejecting roller is installed at a recording paper exit of the fixing device 100.
The recording paper which the toner image has been transcribed thereon enters a nip portion between the fixing roller 100 and the pressing roller 101 through the entrance guiding plate 108. By pressing and heating at the nip portion, the toner image is fixed on the recording paper, and then the recording paper is ejected through a paper-ejecting roller 109. When the recording paper passes the nip portion, the toner attached on the fixing roller 100 is removed by the cleaning pad 105.
The surface temperature of the fixing roller 100 is detected by the temperature detecting device 103. According to a detected result of the temperature detecting device 103, a heating control of the fixing heater 102 is performed by a controlling device (not shown).
Conventionally, an infrared heater or a halogen heater is used as the fixing heater 102 in the fixing device, and a structure constituted of the fixing roller 100 and the pressing roller 101 pair is most common. The pressing roller 101 is a structure that is made by forming silicon rubber on a core bar and then a mold-releasing layer (such as Teflon, registered Trademark) is formed on the silicon rubber, so that the heat capacity of the pressing roller 101 becomes larger. At the beginning of heating, the pressing roller 101 does not get warm easily, causing incomplete fixing problems. Furthermore, there is also a demerit that parts have high cost. In addition, in order to assure a suitable nip width of the nip portion between the fixing roller 100 and the pressing roller 101, a large pressing force (pressure) is required. Because of the pressure, stress will act on the recording paper, causing fixing wrinkle problems.
Instead of the pressing roller 101, a nip portion is formed by pressing a sheet to be in contact with the fixing roller 100. For example, a fixing device, where a non-fixed image is fixed on the recording paper by making the recording paper pass the nip portion, is well studied and researched. By the above structure, the sheet can be easily warmed up, and the nip width can be also assured.
However, regarding the fixing device using the sheet material, the heat capacity of the nip portion is small. Therefore, when the recording paper enters the nip portion, the heat will dissipate suddenly. At this time, the temperature detected by a thermistor might cause the temperature at the nip portion to be lowered and incomplete fixing might occur at the rear end of the recording paper because the position of the thermistor is located outside the nip portion and the response time of the detection is slow.
According to the foregoing description, an object of this invention is to provide a fixing device and an image forming device. The fixing device is composed of a heat conducting unit and a sheet. When a recording paper enters a nip portion, the heat from the sheet being suddenly dissipated can be prevented.
According to the objects mentioned above, the invention provides a fixing device, comprising a heat conducting unit, having a heating source, and being heated by the heating source; and a sheet that is in contact with and presses the heat conducting unit. One end of the sheet is mounted on a fixing frame that is located at an upstream side of a transporting direction of a recording paper, and the other end of the sheet is in contact with the heat conducting unit under a pressing status. When the recording paper onto which a toner image has been transcribed is passed through a nip portion between the sheet and the heat conducting unit, by pressing and heating, the toner image is fixed on the recording paper. Furthermore, a heating unit of line shape is laid on a back surface of the sheet at a region in the vicinity of the nip portion. According to the above structure, because the sheet is heated by the heating unit, the fixing device can be in a fixable status quickly. Additionally, when the recording paper passes the nip portion between the heat conducting unit and the sheet, the temperature of the nip portion can be prevented from being suddenly reduced.
In the above fixing device, the heating unit can be laid parallel to a transporting direction of the recording paper. According to this structure, the strength of the sheet becomes robust and the pressure against the heat conducting unit can become more stable. Furthermore, because the central portion of the sheet can be bent in the up-and-down direction, the sheet can be changed to a suitable shape to meet the shape of the heat conducting unit.
Alternatively, the heating unit can be laid in a grid shape. According to this structure, the strength of the entire sheet becomes more robust and the pressure against the heat conducting unit can become more stable.
The invention further provides an image forming device, comprising a fixing device that has the aforementioned structure and features. According to the above structure, the fixing property can be improved